[flang] Continue semantic checking after name resolution error

When an error occurs in name resolution, continue semantic processing
in order to detect other errors. This means we can no longer assume
that every `parser::Name` has a symbol even after name resolution
completes. In `RewriteMutator`, only report internal error for unresolved
symbol if there have been no fatal errors.

Add `Error` flag to `Symbol` to indicate that an error occcurred related
to it. Once we report an error about a symbol we should avoid reporting
any more to prevent cascading errors. Add `HasError()` and `SetError()`
to simplify working with this flag.

Change some places that we assume that a `parser::Name` has a non-null
symbol. There are probably more.

`resolve-names.cc`: Set the `Error` flag when we report a fatal error
related to a symbol. (This requires making some symbols non-const.)
Remove `CheckScalarIntegerType()` as `ExprChecker` will take care of
those constraints if they are expressed in the parse tree. One exception
to that is the name in a `ConcurrentControl`. Explicitly perform that
check using `EvaluateExpr()` and constraint classes so we get consistent
error messages.

In expression analysis, when a constraint is violated (like `Scalar<>`
or `Integer<>`), reset the wrapped expression so that we don't assume it
is valid. A `GenericExprWrapper` holding a std::nullopt indicates error.
Change `EnforceTypeConstraint()` to return false when the constraint
fails to enable this.

check-do-concurrent.cc: Reorganize the Gather*VariableNames functions
into one to simplify the task of filtering out unresolved names. Remove
`CheckNoDuplicates()` and `CheckNoCollisions()` as those checks is
already done in name resolution when the names are added to the scope.

Original-commit: flang-compiler/f18@bcdb679405906575f36d3314f17da89e3e89d45c
Reviewed-on: https://github.com/flang-compiler/f18/pull/429
Tree-same-pre-rewrite: false